"thousands of weeds in my mulch"

I had a client call and complain that their beds were loaded w/ weeds. Unfortunately w/ all of the rain we've had, the Maple trees have been sprouting up all over (including valleys of roofs and even some gutters). I explained to the client that its due primarily to the over 8" of rain this month alone, and that we'd take care of it for a fee. He tried to tell me that is a direct result of weed seeds in the mulch! ANyway, since I do a lot of work in his neighborhood, I agreed to spray down his beds w/ round-up. It'll cost me about 1 - 3 gal backpack sprayer and 1 hour of my time including travel.
I will definately let this client know that this will NOT be an on-going recurrence. He paid his bill promptly, and was disappointed over something out of my control (Mother Nature).
The majority of my other clients are well enough educated to realize that this year has been a fluke!
How would you all have handled this situation??

Depending on how big of an account he is to you. If its just a small one id probably drop his a## because he will always want every little thing fixed for free now. If its a large account then i would try to bargain with him and cut the price in half.